Crombé, Kristel

TELESEA, G

TOKAR, M

UNTERBERG, B

Van Oost, Guido

Verdoolaege, Geert

ZAGORSKI, R

English

Ghent University Academic Bibliography

Journal of Nuclear Materials 337–339 (2005) 361–365www.elsevier.com/locate/jnucmatPreliminary study of the inﬂuence of DED on carbonradiation and transport in the TEXTOR tokamakG. Telesca a,*, K. Crombe´ a, M. Tokar b, B. Unterberg b,G. Verdoolaege a, R. Zagorski c, G. Van Oost aa Department of Applied Physics, Ghent University, Rozier 44, B-9000 Gent, Belgiumb Institut fuer Plasmaphysik, Forschungszentrum Juelich GmbH, EURATOM Association, D-52425 Juelich, Germanyc Institute of Plasma Physics and Laser Microfusion, P.O. Box 49, 00-908 Warsaw, PolandAbstractChanges on transport and radiation properties of intrinsic carbon under the inﬂuence of the Dynamic ErgodicDivertor depend on the modal composition of the induced error ﬁeld. For 12/4 mode operation, signiﬁcant carbonscreening is not observed up to half of the nominal DED maximum current achieved so far. On the contrary, for 3/1 DED operation the very eﬃcient mechanism of expulsion of highly ionized carbon ions, observed to occur locallyat the foot prints of the 3/1 helical structure and explained in terms of frictional forces, leads to a global carbon decon-tamination of the central plasma. This result has been established using a new multi-chord diagnostic, which allows todistinguish global from local carbon behavior. 2004 Elsevier B.V. All rights reserved.PACS: 52.25.Vy; 52.25.Fi; 52.70.KzKeywords: Carbon impurity; Impurity screening; Spectroscopy; Stochastic boundary; TEXTOR1. IntroductionIn this paper experimental data on carbon behaviorunder the inﬂuence of the dynamic ergodic divertor,DED, on TEXTOR [1] are reported and discussed.The DED is mainly intended to optimize heat and par-ticle exhaust through the formation of a magnetic diver-tor and of a stochastic layer at the plasma edge. Thestudy of the response of intrinsic carbon – in terms oftransport and radiation properties – to a change in the0022-3115/$ - see front matter  2004 Elsevier B.V. All rights reservdoi:10.1016/j.jnucmat.2004.10.108* Corresponding author. Tel.: +49 2461 615174; fax: +492461 613331.E-mail address: g.telesca@fz-juelich.de (G. Telesca).edge ﬁeld structure can provide a key element to assessthe viability of the DED concept to reactor grade exper-iments. The data refer to the ﬁrst experiments with DEDoperating at m/n = 12/4 and at m/n = 3/1. For the 12/4mode, engineering problems have limited so far the levelof the currents in the DED coils and by consequence thelevel of the associated error ﬁeld, which at presentreaches about half of its nominal value, see Ref. [2].Since the decay length of the error ﬁeld is a strong func-tion of m and it is short for m = 12 [3], the external ﬁeldin 12/4 operation aﬀects only the very edge of the plasmaand its action is marginal if the DED currents are onlyhalf of their maximum nominal value. In this sense,the data reported for the 12/4 mode have to be consi-dered very preliminary. On the other hand, the globaled.Fig. 1. Lines of sight of the UV carbon diagnostic. Channel 1 isnot yet operational.362 G. Telesca et al. / Journal of Nuclear Materials 337–339 (2005) 361–365diagnostic capability of TEXTOR during the 12/4 cam-paign and partly also during the subsequent 3/1 cam-paign, was modest (the 12/4 campaign was the ﬁrstexperimental campaign after a long shut down). Forexample, the Electron Cyclotron Emission diagnosticfor the measurement of the electron temperature proﬁlewas not routinely operational during the 12/4 modeexperiments as well as Zeﬀ and the charge exchangerecombination spectroscopy, CXRS. The aim of thepresent work is to establish criteria useful for the studyof the eﬀects of the DED on transport and radiationof the intrinsic carbon. The speciﬁc diagnostic used forthis study is a new eight channel spectroscopic carbondiagnostic, which has been designed to monitor thechange in carbon ﬂux and density under the action ofDED. Indeed, as experimentally shown by the ErgodicDivertor in the tokamak Tore Supra [4] and theoreti-cally predicted [5], at the onset of the perturbing radialﬁeld one should expect the increase of impurity screen-ing as well as the increase of the ratio of power radiatedto impurity density in the plasma core. After a shortdescription of the new carbon diagnostic (Section 2),the data are presented in Section 3 and discussed inSection 4.2. UV and VUV carbon diagnosticsThe brilliances of a CIII line and of a CV line in theUV range (at 229.7nm and at 227.1nm, respectively),are detected simultaneously along each of the eight linesof sight (from channel 2–9; channel 1 is not yet opera-tional) shown in Fig. 1 [6]. Four channels (N. 2,3,4,5)intercept the divertor plates in the high ﬁeld side wherethe DED coils are located, the other four can provideinformation of carbon behavior far from carbonsources. According to the design of the diagnostic, allthe 16 (18) signals will be detected simultaneously at asampling rate of 20kHz. At present, however, the diag-nostic can operate only at reduced performance that isthe CIII and CV signals are detected simultaneouslyalong each of the eight channels but with a time resolu-tion of 100ms. Unfortunately, a technical problem pre-vented so far the accomplishment of a reliable relativecalibration of the eight cannels. This means that the rel-ative intensities of the 16 signals are not related to therelative photon ﬂuxes. By consequence, we have to limitourselves to the comparison of the time evolution of the16 signals, each independently of the others. A secondoperational mode of the diagnostic consists in detectingcontinuously in time the CIII and CV signals from onlyone out of the eight channels, at the cost, however, ofloosing information from the other seven channels. Inthe toroidal sector of our diagnostic, for 12/4 mode ofoperation the foot prints of the strikes zones are locatedoutside the small areas (a few square centimeters each)of the divertor plates intercepted by the lines of sight[7]. Therefore, for standard DC operation the carbonsignals represent average quantities, not localized events.Only in case of oscillation of the magnetic structure atthe 2Hz (see Section 3) channel 4 intercepts one footprint of the magnetic disturbance. For 3/1 DED DCoperation, channels 2 and 3 intercept the foot print ofthe strike zones, while the others are far from localizedevents. Two further channels (channels A and B inFig. 1) have been added at the end of the 12/4 campaign(hence available only during the 3/1 campaign) for thedetection of CIII lines emitted in the vicinity of and onthe ALT limiter. In this study, we make comparison be-tween our UV carbon signals and VUV carbon signalsmeasured at a diﬀerent location in toroidal and poloidalangles. The relative location of the two diagnostics issuch that, in general, the helical structure of the distur-bance does not correlate the signals of the two diagnos-tics, except with 12/4 operation, for which channel 2 ofthe UV diagnostic sees the same foot print seen by theVUV diagnostic.3. Experimental data3.1. m = 12, n = 4 modeWhen the DED is operated in 12/4 mode, the mag-netic surface q = 3 is positioned as close as possible tothe DED coils. This shift is followed by the onset of01 CIII UV02 CV UV1.2 1.6 2 2.4 2.8-40DED currenttime [s][kA][a.u.][a.u.]Fig. 2. TEXTOR shot no. 93087. Time traces (from the top) ofthe DED current and of the CIII and CV signals in UV forDED DC 12/4 operation.0.51CIII HAS1 CV UV-2-1 DED current2.62.8 Prad22.5CIII UVne[a.u.]a.u.][kA][10m]19-3[10W]5G. Telesca et al. / Journal of Nuclear Materials 337–339 (2005) 361–365 363the DED currents. In spite of the marked eﬀects inducedon the edge magnetic structure and on the recycling atthe four stripes of the strike zones (see Ref. [2]), theinﬂuence of DED on carbon signals measured far fromthe stripes (see Section 2) is not very pronounced. CIand CII signals in the visible, measured on the equato-rial plane of the divertor targets on the HFS, increaseby about 20–30%, as well as the CIII signal measuredin the VUV. Comparable behavior is observed for ourUV CIII signals measured along the eight chords. CV re-mains stationary both for the VUV line and for the eightchannel diagnostic (see Fig. 2). The spectroscopic signalsfrom the ALT toroidal limiter on the LFS are generallynot available, due to the shift of the plasma column to-ward the HFS prior to the onset of the DED currents.However, indirect information on the total impurityﬂuxes can be derived from bolometric measurements(four cameras), which indicate only a minor increase inthe total radiated power at the onset of DED currents.Bremsstrahlung signals are consistent with a slight in-crease of Zeﬀ. In one series of discharges with DED cur-rents IDED = 7kA in which the strike points were swept0.40.6 CIII UV33.5CV UV3020 CIII VUV1.4 1.8 2.2 2.62040CV VUV0.51-505 DED currenttime [s][a.u.][a.u.][kA][a.u.][a.u.][a.u.]CII at ALTFig. 3. TEXTOR shot no. 92847. Time traces (from the top) ofthe DED currents, CIII signal UV on channel 4, CIII signalVUV, CII on ALT, CV in UV on channel 4, CV in VUV forDED 12/4 mode at 2Hz oscillation.at a frequency of 2Hz, the eight channel diagnostic wasoperated in continuous mode (see Section 2) with CIIIand CV signals from channel 4. The UV CIII signal in-creases by more than 50% while CV signal oscillatesmoderately (see Fig. 3). The VUV signals show the in-crease of CIII by about 30% and the constancy in theCV line. The diﬀerent response of the two diagnosticsderives from the lack of correlation via the helical struc-ture (VUV is far from the stripes and channel 4 of theUV is on one stripe). In this case, VUV represents theglobal response of CIII and CV, while UV representsthe local response to the action of the magneticperturbation.3.2. m = 3, n = 1 modeSince in this conﬁguration the error ﬁeld penetratesdeeper into the plasma, there is no need for shiftingthe plasma column toward the HFS. Due to the presenceof a strong Fourier component m = 2 in the 3/1spectrum of DED on TEXTOR (see Ref. [8]), at a cer-tain value of IDED, which is about 0.8kA for DC oper-ation and nearly a factor of two higher for AC, the 2/102 32.22.4 Zefftime [s][011.2 1.24 1.2801CIII UV2.9 2.94 2.98CV UVCIII UVtime [s] time [s][a.u.][a.u.]CV UV(a)(b)Fig. 4. (a) TEXTOR shot no. 94454. Time traces (from the top)of the central line average density, DED current, Prad, CIII andCV in the UV, and average Zeﬀ for DED DC 3/1 operation. Atabout t = 1.9s the 2/1 tearing mode locks to the ﬁeld. (b)TEXTOR shot no. 94454. Intensity of CIII and CV signals inthe eight channels at DED current IDED = 0 and at IDED = 2kAfor 3/1 operation. Channel 2 appears ﬁrst and channel 8 last inthe series of eight signals.364 G. Telesca et al. / Journal of Nuclear Materials 337–339 (2005) 361–365mode locks to the error ﬁeld. This locking is correlatedwith a noticeable increase in plasma–wall interactionand loss of particles and energy from the plasma. Typi-cally (see Fig. 4(a)), in IDED current up experiments thecarbon lines remain nearly unchanged up to the lockingof the 2/1 mode. From this time on, the total radiatedpower, Prad and the CIII signals from the eight channeldiagnostic increase in time, while simultaneously the CVsignals decrease. The two CIII signals on the LFS de-crease; Zeﬀ tends to decrease only slightly, but systemat-ically in all discharges. CVI signals from CXRS areconsistent with a reduction of about 20% of the carbondensity in the plasma core [9]. The level of the increase ofthe CIII signals from the eight channel diagnostic, aswell as that of decrease of the CV signals is, however,quite diﬀerent for diﬀerent channels. As can be seen ina zoom of the CIII and CV signals (see Fig. 4(b)), theCIII line increases by a factor of 5 in the ﬁrst two chan-nels (channels 2 and 3) while increases only about by10% on the other cannels. Similar, but opposite in sign,is the behavior of CV line, which decreases by nearly afactor of two on channels 2 and 3 and only by about10% on the others.0 0.5 1.0 1.5 2.0024681012 TEXTOR # 94454channel 2channel 8I / kAI CIII/ ICV, normalizedDEDFig. 5. The ratio CIII/CV (normalized for IDED = 0) as afunction of IDED for channel 2 pointing at the zone of stronglocal plasma–wall interaction, and for channel 8.4. DiscussionTo a very ﬁrst approximation one can assume theﬂuxes of C3+ ions to be proportional to CIII signal(emitted by C2+ ions) and the ﬂuxes of C5+ ions to beproportional to the CV signal (emitted by C4+ ions). In-deed, although the ionizations per photon for the CIIIline and for the CV line are function of the electron tem-perature [10], in the presence of temperature gradients,as in the plasma edge of tokamaks, the ions of any givenionization level tend to be located at a radial position inwhich the value of the electron temperature is about onehalf of their ionization potential [11]. This means, forexample in the case of CIII line, that the ratio betweenthe number of photons emitted by C2+ ions (the CIII sig-nal) and the number of ionization events from C2+ ionsto C3+ ions (the ﬂux of C3+ ions) can be assumed to beconstant. For DC 12/4 operation one has to compare theaverage increase with the onset of DED of about 25–30% of CIII signals in the area of the divertor plate inthe HFS with the decrease of the signals in the area ofthe toroidal limiter ALT in the LFS, which is assumedto be of similar level. Considering the ratio of the surfaceof the divertor to the surface of the ALT limiter (about2) one arrives at a modest total increase of C3+ ﬂuxes ofabout 10–15%. This rough estimate is consistent with theobserved marginal increase of Prad, which is relatedessentially to the densities of Li-like C3+ ions. Apartfrom the small oscillation observed on the channel 4for the 2Hz experiment, the ﬂux of the C5+ ions gener-ally does not change in the channels of the UV diagnos-tic as well as for the VUV diagnostic for DC operation.We can conclude, therefore, that the ratio of the CIII toCV signals increases slightly, in average, for 12/4 DEDmode. Since the C3+ ions are located in the edge per-turbed zone of the DED ﬁeld, while C5+ ions are locatedin the unperturbed zone well inside the q = 3 surface, thechange of the ratio CIII/CV can be taken as a qualitativeﬁgure of merit for the screening eﬃciency of the DEDﬁeld. The higher becomes this ratio the better is theDED screening eﬃciency.In 3/1 mode DC operation of the DED, when the 2/1tearing mode becomes locked a localized extremely rele-vant event occurs in correspondence of channels 2 and 3of the UV diagnostic (see Fig 4 and Fig 4(a)). In fact,channels 2 and 3 intercept the foot print of the strikezones of the 3/1 structure generated by the DED cur-rents [12]. While on these two channels the ratio CIII/CV increases up to a factor of 10 for DED currents upto 2kA, see Fig. 5, on the other channels this ratio in-creases up to a factor of two. This diﬀerence makes aclear distinction between local events and global re-sponse of the plasma. In line with the interpretation ofthe action of the Ergodic Divertor of Tore Supra onimpurities [4] given in Ref. [5], the experimental ﬁndingsof the DED eﬀects can be explained as follows. Them/n = 3/1 ﬁeld structure generated by DED (long decaylength, see Section 1) connects the plasma core with theedge plasma and causes a plasma ﬂow along ﬁeld lines.Due to frictional forces (charge dependent), highly ion-ized impurities are selectively dragged and expelled,resulting in a global plasma decontamination of non-recycling impurities. On the other hand, the low ion-ized carbon ions, which are mostly responsible for thetotal radiated power, are hardly aﬀected by this expul-sion mechanism. Modeling of carbon transport madewith the 2D ﬂuid code TECXY [13], based on thetransport mechanism used in [4], reproduces the mainfeatures of the experimental data reported in thispaper.G. Telesca et al. / Journal of Nuclear Materials 337–339 (2005) 361–365 3655. ConclusionAlthough for 12/4 DED mode of operation the ratioCIII/CV tends to increase slightly, it would be prema-ture to asses the existence of a carbon screening eﬀect.This loss of screening might depend on the low penetra-tion together with the low level of the radial magneticﬁeld strength generated so far by the DED currents.In 3/1 DED mode, plasma decontamination of highlyionized carbon ions has been clearly identiﬁed. Thisdecontamination, which does not involve low ionizedcarbon ions, hence the power radiated, can be explainedin terms of plasma ﬂow along ﬁeld lines and frictionalforces. This mechanism of plasma decontamination ofhighly ionized non-recycling ions is, however, associatedwith losses on the particle and energy channels.AcknowledgmentsThe support by the FWO (Fund for Scientiﬁc Re-search-Flanders) is acknowledged.References[1] K.H. Finken et al., Nucl. Fus. 38 (1998) 515.[2] M. Lehnen et al., these Proceedings. doi:10.1016/j.jnucmat.2004.10.100.[3] S.S. Abdullaev et al., Phys. Plasmas (6) 153.[4] Ph. Ghendrih et al., Plasma Phys. Control. Fus. (1996)1653.[5] M.Z. Tokar et al., Plasma Phys. Control. Fus. 39 (1997)569.[6] K. Crombe´ et al., 29th EPS Conference Montreux (2002),ECA Vol. 26B, P1-2.056.[7] W. Jakubowski et al., Nucl. Fus. Vol. 6 (44) (2004).[8] Y. Liang et al., paper P1-126 presented at the 31st EPSConference, London, 2004.[9] M. Von Hellermann et al., paper P-118 presented at the31st EPS Conference, London, 2004.[10] A. Pospieszczyk, in: R.K. Janev, H.W. Drawin (Eds.),Atomic and Plasma-Material Interaction Processes inControlled Thermonuclear Fusion, Elsevier Science B.V.,1993.[11] G. Telesca et al., Nucl. Fus. 34 (1994) 625.[12] S.S. Abdullaev, private communication.[13] H. Gerhauser, Zago´rski, et al., these Proceedings.doi:10.1016/j.jnucmat.2004.07.059.

Preliminary study of the influence of DED on carbon radiation and transport in the TEXTOR tokamak

https://biblio.ugent.be/publication/350014/file/1128696

Preliminary study of the influence of DED on carbon radiation and transport in the TEXTOR tokamak

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography